The oil and gas industry incurs substantial operating costs to drill wells in the exploration and development of hydrocarbon resources. The cost of drilling wells may be considered to be a function of time due to the equipment and manpower expenses based on time. The drilling time can be minimized in at least two ways: 1) maximizing the Rate-of-Penetration (ROP) (i.e., the rate at which a drill bit penetrates the earth); and 2) minimizing the non-drilling rig time (e.g., time spent on tripping equipment to replace or repair equipment, constructing the well during drilling, such as to install casing, and/or performing other treatments on the well). Past efforts have attempted to address each of these approaches. For example, drilling equipment is constantly evolving to improve both the longevity of the equipment and the effectiveness of the equipment at promoting a higher ROP. Moreover, various efforts have been made to model and/or control drilling operations to avoid equipment-damaging and/or ROP-limiting conditions, such as vibrations, bit-balling, etc.
Many attempts to reduce the costs of drilling operations have focused on increasing ROP. For example, U.S. Pat. Nos. 6,026,912; 6,293,356; and 6,382,331 each provide models and equations for use in increasing the ROP. In the methods disclosed in these patents, the operator collects data regarding a drilling operation and identifies a single control variable that can be varied to increase the rate of penetration: In most examples, the control variable is Weight On Bit (WOB); the relationship between WOB and ROP is modeled; and the WOB is varied to increase the ROP. While these methods may result in an increased ROP at a given point in time, this specific parametric change may not be in the best interest of the overall drilling performance in all circumstances. For example, bit failure and/or other mechanical problems may result from the increased WOB and/or ROP. While an increased ROP can drill further and faster during the active drilling, delays introduced by damaged equipment and equipment trips required to replace and/or repair the equipment can lead to a significantly slower overall drilling performance. Furthermore, other parametric changes, such as a change in the rate of rotation of the drill string (RPM), may be more advantageous and lead to better drilling performance than simply optimizing along a single variable.
Because drilling performance is measured by more than just the instantaneous ROP, methods such as those discussed in the above-mentioned patents are inherently limited. Other research has shown that drilling rates can be improved by considering the Mechanical Specific Energy (MSE) of the drilling operation and designing a drilling operation that will minimize MSE. For example, U.S. Pat. Nos. 7,857,047, and 7,896,105, each of which is incorporated herein by reference, discloses methods of calculating and/or monitoring MSE for use in efforts to increase ROP. Specifically, the MSE of the drilling operation over time is used to identify the drilling condition limiting the ROP, which often is referred to as a “founder limiter.” Once the founder limiter has been identified, one or more drilling variables can be changed to overcome the founder limiter and increase the ROP. As one example, the MSE pattern may indicate that bit-balling is limiting the ROP. Various measures may then be taken to clear the cuttings from the bit and improve the ROP, either during the ongoing drilling operation or by tripping and changing equipment.
Recently, additional interest has been generated in utilizing artificial neural networks to optimize the drilling operations, for example in U.S. Pat. Nos. 6,732,052, 7,142,986, and 7,172,037. However, the limitations of neural network based approaches constrain their further application. For instance, the result accuracy is sensitive to the quality of the training dataset and network structures. Neural network based optimization is limited to local search and conventionally has difficulty in processing new or highly variable patterns.
In another example, U.S. Pat. No. 5,842,149 disclosed a close-loop drilling system intended to automatically adjust drilling parameters. However, this system requires a lookup table to provide the relations between ROP and drilling parameters. Therefore, the optimization results depend on the effectiveness of this table and the methods used to generate this data. Consequently, the system may lack adaptability to drilling conditions that are not included in the lookup table. Another limitation is that downhole data is required to perform the optimization.
While these past approaches have provided some improvements to drilling operations, further advances and more adaptable approaches are still needed as hydrocarbon resources are pursued in reservoirs that are harder to reach and as drilling costs continue to increase. Further desired improvements may include expanding the optimization efforts from increasing ROP to optimizing the drilling performance measured by a combination of factors, such as ROP, efficiency, downhole dysfunctions, etc. Additional improvements may include expanding the optimization efforts from iterative control of a single control variable to control of multiple control variables. Moreover, improvements may include developing systems and methods capable of recommending operational changes during ongoing drilling operations.